1. Field of the Invention
The present invention relates to a drive shaft used in an automotive drive train or the like, and more particularly, relates to a drive shaft comprising a shaft and a constant velocity joint.
2. Description of the Related Art
A power transmission device for automobiles or the like normally transmits a driving force through a drive shaft comprising a shaft portion and a constant velocity joint. The shaft portion undergoes induction heat treatment or the like to increase its strength, which results in the formation of a hardened layer up to a certain depth from the surface while leaving a non-hardened portion at the shaft core. Both ends thereof are in splined engagement with interior members of the constant velocity joint (for an example, refer to Japanese Patent Laid-Open Publication No. 2000-240669, paragraphs 2 to 10 and FIG. 1). FIG. 3 shows an example of such a drive train, wherein a shaft portion 1 has boot mounting parts 3, 5 and extensions 4, 6 respectively connected and coaxially formed with both ends of a shaft body 2.
The inboard-side extension 4 of the shaft portion 1, for example, is connected to a driving-side member of a side gear or the like in a differential, via a tripod-type sliding constant velocity joint 30. Meanwhile, the outboard-side extension 6, for example, is connected to a driven-side member of a hub or the like in a driven wheel, via a ball-type fixed constant velocity joint 35. Boots 33, 38 made from soft rubber, resin or the like are provided in order to seal the interiors of the constant velocity joints 30, 35 against dirt, water and so forth from outside. More specifically, the boots 33, 38 are respectively provided between the constant velocity joints 30, 35 and the boot mounting parts 3, 5 on both ends of the shaft body 2 of the shaft portion 1. One end of each of the boots 33, 38 overlay an opening on each of exterior members 31, 36 of the constant velocity joints 30, 35, and is attached using bands 34a, 39a so as to form an airtight seal. Another end is attached via the boot mounting parts 3,5 using bands 34b, 39b so as to form an airtight seal as well. In order to fixedly mount the boots 33, 38 and ensure sealing, boot grooves 3a, 5a are formed on the boot mounting parts 3, 5.
As shown in FIG. 4, the inboard-side extension 4 comprises a cylindrical intermediate portion 4a, as well as an end 4b in spline engagement with an interior member 32a of the constant velocity joint 30. The intermediate portion 4a has an outer diameter smaller than the boot mounting part 3 and the end 4b. Both ends of the intermediate portion 4a are connected to the boot mounting part 3 and the end 4b by tapered portions 4c, 4d. The value for the outer diameter of the intermediate portion 4a of the extension 4 is the minimum necessary for obtaining a required transmitted torque. Formed on an opening-side of the inner surface of the exterior member 31 of the constant velocity joint 30 is a flank 31a that faces outward and leaves a slight clearance with the intermediate portion 4a. The flank 31a is formed in order to avoid interference with the intermediate portion 4a of the extension 4 at a maximum shaft intersection angle between the extension 4 and the exterior member 31.
Likewise, as shown in FIG. 5, the outboard-side extension 6 comprises a cylindrical intermediate portion 6a, as well as an end 6b in spline engagement with an interior member 37 of the constant velocity joint 35. The intermediate portion 6a has a diameter smaller than the boot mounting part 5 and the end 6b. Both ends of the intermediate portion 6a are connected to the boot mounting part 5 and the end 6b by tapered portions 6c, 6d. Similar to the extension 4, the value for the outer diameter of the intermediate portion 6a of the extension 6 is the minimum necessary in terms of strength. Formed on an opening-side of the inner surface of the exterior member 36 of the constant velocity joint 35 is a flank 36a that faces outward, and also leaves a slight clearance with the intermediate portion 6a at a maximum shaft intersection angle.
For both the extension 4 and the extension 6, the position of a boundary between the intermediate portions 4a, 6a and the base side of tapered portions 4c, 6c at the maximum shaft intersection angle is set somewhat apart from outer edges 31b, 36b of the flanks 31a, 36a of the constant velocity joints 30, 35 and set towards the boot mounting parts 3, 5 sides.
As mentioned above, a hardened layer is formed up to a certain depth from the surface while leaving a non-hardened portion at the shaft core to increase the strength of the shaft in the drive shaft. In order to obtain a maximum overall increase in strength, the depth of heat treatment, t/R (where, t: depth of hardened layer, and R: radius of heat treated member), must be set to a certain optimum value. For example, alloy steel used in the shaft differs depending on the material, but the optimum heat treatment depth t/R is within the range of 0.55 to 0.8. In other words, the static torsional strength improves as the heat treatment depth t/R increases; although the fatigue torsional strength falls once the heat treatment depth t/R exceeds a certain limit. Therefore, the heat treatment depth t/R must be set to a value smaller than 0.8. The outer diameters of the intermediate portions 4a, 6a of the extensions 4, 6 are set to the minimum values necessary for obtaining the required transmitted torque. Meanwhile, the boot grooves 3a, 5a are formed on the boot mounting parts 3, 5 to attach the boots 33, 38, and making the outer diameter of the bottom portion of the boot grooves 3a, 5a smaller than the outer diameter of the intermediate portions 4a, 6a would adversely affect strength. Consequently, the outer diameters of the boot mounting parts 3, 5 forming both sides of the boot grooves 3a, 5a are larger than the outer diameters of the intermediate portions 4a, 6a, which results in an outer diameter of the shaft portion 1 that varies among the intermediate portions 4a, 6a and the boot mounting parts 3, 5. Moreover, splines formed on the ends 4b, 6b necessitate an outer diameter larger than that of the intermediate portions 4a, 6a for the sake of ensuring strength, which also means the outer diameter of the shaft portion 1 varies among the intermediate portions 4a, 6a and the ends 4b, 6b as well.